1. Field of the Invention
This invention relates to the field of glass treatment using hot liquid baths, and in particular to shaping, tempering and annealing glass in a bath of molten salt.
2. Description of the Prior Art
Heating and chemical tempering of glass parts using hot liquid baths is not new to the art of glass forming. Such baths, usually comprising molten salt or metal, are beneficial in that thermal inertia allows the mass of liquid to maintain a relatively constant temperature. In addition, simultaneous heating and chemical treatment can be accomplished in the bath, the larger cation of the salt bath being exchanged with the smaller cation in the glass surface (e.g., potassium for sodium or sodium for lithium). All that is necessary for heating purposes is that the bath composition have a melting point lower than the softening point of the glass, and not disassociate or boil at the temperatures necessary for forming.
Although the use of molten liquid apparatus for heating has been known to have the foregoing advantages, there are also certain problems. For example, the sudden temperature change encountered when a glass part is moved from the relatively cool ambient air into the relatively hot molten salt bath results in uneven expansion of the glass and can cause cracking. To combat such uneven heating, glass blanks are often preheated before immersion in the molten salt bath. A similar temperature difference problem is encountered upon removal. While the molten salt is liquid at the temperature of the bath, the salt becomes a solid at the lower ambient temperature. The salt or metal bath composition clings to the glass as it solidifies and must be removed. Where bending is accomplished in a molten salt or metal bath, the hardening bath composition tends to adhere to the bending apparatus as well, when removed from the vat of molten bath composition and cooled.
Bending and handling apparatus which must be repeatedly dipped into a molten salt bath is soon corroded. Materials which are not easily corroded, such as stainless steel, are expensive. Therefore, there is a need to minimize the handling apparatus associated with such dipping operations.
The usual mechanical devices for forming, shaping and supporting glass do not work adequately in a molten salt environment. In addition to the foregoing problems, the bath liquid is obviously more viscous than is the air, and operations must be conducted beneath the surface of the liquid which is at best translucent and at worst opaque. When these disadvantages are combined with the problems attendant with forming of very thin glass parts, it is not surprising that molten salt forming has not been widely accepted in the industry.
In furnace heating processes, the prior art has employed porous members where a gas, such as a blast of cooling air, was directed against softened glass while supported in its intended shape. The porous members were sometimes built rigidly as parts of molding members, whereby the cooling air could be passed directly through the mold face to the glass. Another example of such use of porous members has been where vacuum heads were used to pick up and transfer soft glass parts. In this situation, of course, the gas was withdrawn through the porous member, rather than supplied.
Flexible porous members have been used to support softened glass blanks. Inasmuch as a flexible support will sag into a parabolic arc under the weight of the glass, such supports have been employed to pre-form softened glass into roughly the shape to be molded, for example by blow forming through the flexible porous member.
Notwithstanding the foregoing uses of flexible coverings for glass, the usual prior art forming technique comprises use of forming members which contact the glass at point contact rather than spreading the contact pressure over a surface. Where a curved male/female mold is closed on a flat glass blank, contact and surface marring occurs at one point on the male member and two points on the female member. As the mold closes further, the glass sheet undergoes stretching forces between the male and female contact points as well as compressing forces between the female contact points. In other words, forces are exerted along the plane of the glass sheet, tending to stretch and ripple the soft glass. Stretching and rippling in the glass means variations in thickness, which variations are undesirable in glass to be laminated and in optical applications.
Flexible porous members such as mesh screens have not heretofore been entirely successful in glass treatment in hot liquid baths. Mesh screens, and in fact porous members in general, compound the problem of adherence of the solidifying liquid bath composition to dipped apparatus such as the mold, the glass and especially the screen. In the prior art, relatively low temperature baths have been thought necessary to reduce the adherence of the bath composition to the glass and/or mold. It will be appreciated that this approach attempted to prevent hardening of the bath composition until the formed part could be removed from the mold and the bath composition removed. Alternatively, non-wetting solutions have been attempted whereby the bath composition would hopefully not cling to the glass or mold.
The present invention employs the mesh screening not only as a support, as was successful in furnace-type environments, but as a forming member in a molten salt bath forming technique. A very fine mesh is used to support and protect the glass blank before, during and after forming. The mesh itself acts as one of two forming members, bending the softened glass over a form, and evenly distributing forming forces over the entire surface of the glass. The extreme temperature change between ambient and the temperature of the hot liquid is ameliorated by the presence of the fine mesh screen, through which the hot liquid can move only slowly. The fine mesh protects the glass surface from marring due to mechanical interaction with the form, and prevents surface checking due to sudden changes in temperature when the glass is inserted and/or removed from the molten salt bath.
Upon removal, the molten salt indeed adheres to the fine mesh screen. However, the mesh is sufficiently fine and is held sufficiently close to the glass that, upon the mesh being removed, the hardened molten salt is virtually all carried away.